This invention relates to an improved nonpneumatic tire which will be used primarily as a temporary spare tire. The improved tire is designed to replace the conventional pneumatic spare tire.
Nonpneumatic tires are old in the art. Nonpneumatic solid rubber tires were in use prior to pneumatic tires. As vehicle speeds increased and ride characteristics became more important the need for a better tire structure emerged. The invention of the pneumatic tire provided a solution to the problems and limitations of solid tires.
The pneumatic tire is an efficient structure that has endured as a solution to conventional vehicle requirements.
A pneumatic tire is a "tensile structure". Tensile structures always contain a compression member for providing a tensile preload in the tensile member. The tensile member can usually accept no compression and the compression member no tension. In pneumatic tires the cords are the tensile member and the compressed air is the compression member.
The primary drawback of a pneumatic tire is that it is pneumatic. Air contained under pressure can and usually does escape at the most inopportune times at least from the driver's view point. It is aggravating to find that the spare tire which has been lugged around in the trunk for years is also flat.
The present invention has no air under pressure. It is a tire structure that performs similarly to a pneumatic tire without requiring air contained under pressure.
Communication of the tire with the road in the area of the footprint or road contact patch provides the only force input to the vehicle and provides the handling forces as well as load support. A nonpneumatic tire must be designed with these characteristics fundamental to a pneumatic tire in mind. A pneumatic tire has unique flexure and load carrying characteristics. Shock and deflections although occurring locally in the area radially inwardly of the footprint are absorbed globally by the entire tire structure. Cornering characteristics are achieved by a combination of sidewall increases and decreases in tension.
A nonpneumatic tire must be able to withstand shock loads and dissipate the energy absorbed. Unlike the pneumatic tire, the nonpneumatic tire generally absorbs shocks and deflects locally in the area within the footprint or contact patch. The tire being capable of localized deflection must exhibit high dampening characteristics.
The tire in a running condition must be able to dissipate heat. The nature of dampening loads is a form of energy dissipation. Energy absorbed is converted to heat. Heat in turn can affect tire performance and can result in premature tire failure; efficient dissipation of heat is essential. Ideally, energy is only absorbed by the tire in the area radially inward of the footprint so that energy can be removed from such area during the remainder of the tire's revolution. Rubber is a poor conductor of heat.
The thicker the rubber the higher the heat buildup. The heat buildup problem can be reduced to a controlled level by having thin material cross sections with high air circulation.
Urethane tires can operate at temperatures as high as about 93.degree. C. (200 degrees F.). Temperatures higher than 121.degree. C. (250 degrees F.) degrees for prolonged periods will cause a weakening of the structure. If the temperature of the tire is high enough this can lead to premature failure.
In 1917 a nonpneumatic tire called automobile cushion wheel was patented by Samuel Johnstone. The U.S. Pat. No. 1,258,573 describes a multicomponent wheel. The Johnstone wheel comprises a central portion of resilient material, an outer resilient tread portion, and an interposed shock absorbing portion comprising a plurality of crossed webs of resilient material formed with the center and tread portions. Formed in the inner portion of the shock absorbing portion is an annular series of orifices. The orifices were set transversely and slightly overlapping. Each orifice extended across the entire axial width of the shock absorbing portion. A pair of disks were also provided with similar orifices. One disk was positioned on each side of the wheel with orifices aligned with those of the shock absorbing portion. Upon molding one integral unit was formed. The cushion wheel so described was claimed to be of no greater weight than metal or wooden wheels of that era. The tire further eliminated the metal parts used to fasten pneumatic or solid rubber tires to the wooden felly.
In 1923, Louis Ninskle was granted U.S. Pat. No. 1,494,797 for a tire. The invention described a rubber tire with encapsulated air recesses or compartments. The cushioning of the tire was achieved through the use of entrapped air.
These earlier attempts to develop a nonpneumatic tire failed to provide good heat dissipation. As vehicle speeds increased, these concepts were incapable of meeting the needs of the day and simply died out.
In 1989, a patent issued for a nonpneumatic tire suitable for use on vehicles of the current era. The nonpneumatic tire is described in the Palinkos, et al. U.S. Pat. No. 4,832,098. The tire is integrally molded from an elastomeric material to form a unitary structure comprising inner and outer "hoops". The outer hoop is supported and cushioned by a plurality of circumferentially spaced apart planar rib members (ribs) and a planar central web member (web) which connects the hoops at their circumferential centering. The web lies in a plane perpendicular to the rotational axis of the tire. The ribs extend axially along the inner and outer hoops connecting them and the edges of the rib lie along the opposite faces of the web. The planar ribs are undercut at the radial extremes to insure that bending is assured and the ribs buckle only when a critical load is exceeded.
The Palinkas, et al. design requires the use of oppositely directed ribs connected to a central planar web and inner and outer hoops. It is this combination of load bearing that is claimed to provide "ride" and handling characteristics of a pneumatic tire.
The applicants' tire described herein, and subject of this invention is an improvement in nonpneumatic tire design.